Television scanning system



Feb. 22,- 1938. A. McL. NICOLSON I TELEVISION SCANNING SYSTEM Ffled July 7. 1950 a Sheet-Sheet 1 INVENTOR A/exander M LmH Mrs/son.

BY @A/KWM ATTORNEY Feb. 22, 1938. A. Mc| NICOLSQN I 2,108,827

TELEVIS ION SCANNING SYSTEM Filed July '7. 1930 3 Sheets-Sheet 2 His-.- E

Sea/min Gene/afar.

INVENTOR BY A/K WM ATTORNEY Feb; 22, 1938.- A, Mc| NICOLSON 2,108,827

TELEVISION SCANNING SYSTEM Filed July 7. 1930 3 Sheets-Sheet 3 INVENTOR A/exdnaer M Lean Abba/$017.

BY QM/KM ATTORN EY Patented Feb. 22, 1938 UNITED? STATES amass? PATENT orrics TELEVISION SCANNING SYSTEM Alexander McLean NicolsomNew York, N. Y.,ias-

signor to Communication Patents, Inc., New York, N. Y., a corporation of Delaware Application July 7, 1930, Serial No. 466,079 24 Claims. (o1.17s 7 2) 10 Another object of this invention is to facilitate.

the scanning of scenes at a considerable distance from the transmitting apparatus. 1 I i A further object of'the invention is to utilize the non-linear characteristic of a photoelectric cell for obtaining differential illuminating currents in accordance with the light and shade intensities of objects.

A still further object of the invention is to increase the efficiency of scanning apparatus by re-,

generation.

in the electrical transmissionof the images of objects or pictures thereof from a transmitting station to a receiving station in the form of electrical impulses characterized by the light and dark intensities of the objects or pictures, the common manner in which the impulses are obtained is the projection of a searching or scanhing ray'of light on the objects or pictures. The varying intensities of the reflectedlight are intercepted by a photoelectric cell or plurality thereof, to effect the transformation into electrical vibrations, which are transmitted from station to station.

It is well known that the intensity of light decreases rapidly with distance from the source.

t is obvious, therefore, that if objects are to be successfully scanned in the usual manner from a considerable distance, an extremely high power light source is required in order to obtain a sufiicient variation in the intensity of the reflected light upon a photoelectric cell positioned approximately the same distance from the object. Even with such a light source, this method is not desirable, from the convenience standpoint.

. In the present invention, a distant or near scene is scanned by obtaining an image of the scene upon the cathode of a photoelectric cell which has a non-linear characteristic. The focusing of the image on the cathode of thecell 50 will produce a steady total emission of electrons to the anode of the cell, and produce a steady electric current in the input circuit of the photoelectric current amplifier. Although total emission remains substantially constant, the emission is not uniform over, the entire cathode, because the cathode is not uniformly illuminated. This fact, coupled with the fact that the emissivity is not directly proportional to the illumination, forms the basis of this invention.

The point of operation on the photocell characteristic curve is determined by the total reflected light falling on the cathode, and this point may bechanged and controlled by the insertion of light filtersofsmokedglass and the like between the object and the cathode of thecell. 10,

This action is analogous to the biasing of the control grid of a-vacuum tube. V

With the image on the cathode itself, a Search ing ray of light, produced in any well known manner, but preferably, in the form of an elec- 1;.5 trodynamic are along electrode rails in a magnetic field, may be employed for scanning the image. Such scanning systems are disclosed in my co-pending. applications Ser. No. 397,826, filed October '7, 1929 and. Ser. No. 450,025, filed 20 May 5, 1930. Since the intensity of the light of the searching ray is substantially constant, vari ations in the current output of the cell are obtained through the non-linear characteristic of the photoelectric cell. For example, when the 25 scanning ray is projected upon the portion of the cell which is least illuminated by the reflected light from the object originating from the sun or an artificial lighting'system, a certain increase'in the steady value of the photocell current is ob- 3O tained. The amount of this increase is entirely dependent upon the amount of light originally falling upon this area. Now, if the scanning ray moves to another portion or unit area of the image having brighter illuminationpan increase'of photocell current will occur, having a value greater than that caused by the ray falling on the first portion. This variation in current in accordance with the light and dark intensities of the image will reproduce the image at a receiving station.

The characteristic of a photocell is comparable to the grid voltage-plate current characteristic of a three element vacuum tube, that is, it has a double inflection. In the present system the lower bend of this curve is employed similarly to the portion used in a grid leak detector for radio reception. When the scanning ray falls on portions of the cathode having small illumination, small increases in photoelectric current are obtained, while proportionally larger increases 00- our from scanning the brighter portions of the image.- This relation is the same as that found whenan. object is directly scanned by the searching ray.

This invention also contemplates the impression of a portion of the generated photoelectric cell currents after amplification upon the scanning ray producing means to intensify the ray proportionally to the light and shade of the image, thereby amplifying and emphasizing the non-linear emission characteristic of the cell.

The details of the invention will be more fully understood. by reference to the accompanying drawings, irrwhich:v

Figure 1 is a perspective diagrammatic arrangement of a single cathode single anode photoelectric cell scanning system;

Fig. 2 is a cross-sectional view of another embodiment of scanning apparatus, in which a douole anode tube is employed, the cathode of which has a definite curvature.

Figs. 3, 4 and 5 are curves representing the operating characteristics of the photoelectric cell.

Referring to Fig. 1 specifically, a photoelectric cell is shown diagrammatically in the form of a camera. The cell has a cathode 5 and an anode 6, the cathode beingof an opaque material, coated with a light sensitive material such as potassium or the like, well known in the art, while the anode is a metallic ring shown in the form of a square, but which may have any configuration. These elements of the tube which in practice are contained in an evacuated or gas filled transparent envelope, are shown connected in the input circuit of an amplifier system comprising a vacuum tube 7. The photoelectric cell is provided with a potential from a source 8, while the elements in the tube are shunted by a resistance Q for varying the impedance of the input to the vacuum tube 1. The vacuum tube is energized from a filament battery 5 l and a plate potential battery Hi. The output of the battery is con nected through output transformer 3 to an amplifier M, which in turn is connected to transmitting apparatus 45 for transmission of the photoelectric cell currents. The transmission may be either over an antenna system H or wire conductors l8 chosen by the operation of a switch E9. Connected to the transmitter is also a scanning generator 22 for producing a scanning ray for a scanning screen 23. The impulses from the scanning generator are transmitted along with the photoelectric cell currents for synchronizing the receiving screen with the transmitter. It is to be understood that any type of scanning system which provides a sharp exploring ray of substantially constant intensity, may be used to scan the image.

There isalso shown in Fig. l a camera 25 with its pinhole 26, for producing a scanning beam of light, and a focusing lens 28 which may be adjusted to properly focus an object on the o'athode. A light intensity controlling screen is interposed between the lens 28 and the anode B, but the screen may also be placed between the scene and the lens.

An object positioned at O for instance, which may be a near or far scene, may have light proiected upon it from either the sun or artificial sources. A portion of this projected light is refiected through the focusing lens 28, the screen 28, the anode 5 onto the photoelectric cell cathode 5, as shown at I. This image is then scanned by the ray of light produced by the screen 28 and projected through the pinhole 26 of the camera 25. Because of the non-linear characteristic of the cell, varying currents are set up in the input circuit of the vacuum tube 1,

which, after amplification, are transmitted to receiving apparatus.

Referring now to Fig. 3, a portion of a characteristic curve of the photoelectric cell is shown, the co-ordinates of which are light intensity in lumens and photoelectric cell current in microamperes. The complete curve may be found in any treatise on photoelectric cells, and if carried to the saturation point, will be found to have a second inflection in the reverse direction. Only the lower portion of this curve is utilized in the present invention, this portion being comparable to the lower inflection of the grid voltage-plate current characteristic of a vacuum tube. Of course, the upper bend can be employed in the same manner as in detection with vacuum tubeshaving a-positive bias. Assuming an image has been projected on the cathode of a cell of the character shown, a steady flow of current will occur between the elements thereof, depending upon the total amount of light received from the scene to be transmitted. On the curve, the least illuminated area may be represented by the point 3i which will produce a certain amount of photoelectric cell current. Another element of the cathode receiving the highest illumination is shown by the point 32, producing another value of photoelectric cell current. The integration over this curveprovides the steady'current value of thecell output.

In Fig. 4, another representation of this phenomenon of the tube is illustrated. The heavy line 34 on the left hand side of the drawings represents the cathode, while the heavy line 35 on the right hand side of the drawings represents the anode, the drawings being a cross-section of these elements. The varying density lines represent the number of electrons flowing from particular points on the cathode, the drawings illustrating the non-uniformity of emission from the cathodes as the light is projected thereon in different intensities. The total amount of current generated by the cell, however, is a substantially steady value in spite of any changing scenes, the changing scenes causing only a redistribution of activity.

In Fig. 5, these relations are shown by a curve plotted between time and photoelectric cell current. The dotted line 3% represents thesteady value of current obtained by the focussing of the scene on the cathode, while the wavy line 3? is the variation in the photocell current caused by the scanning thereof by the scanning apparatus. The addition of the scanning ray will increase the photoelectric cell current, and a minimum will always be above that of the steady value caused by the reflected light from the scene. 7

Returning now to Fig. 3, which shows how the wavy line 31 of 5 is obtained, we may assume that the light intensity of the scanning ray is equal to the distance a between the vertical parallel lines. That is, the illumination of all. unit areas of the cathode is increased by a consecutively by the ray passing over them. When the arc is projected, therefore, on a dark portion of the image on the cathode, the increase in photoelectric cell current may be considered as the distance between the first set of horizontal lines I). As the ray shifts to a portion which has abrighter illumination, for instance as shown by the middle set of vertical lines, the increase in photoelectric current will be that shown by the distance between the middle set of horizontal lines 0. When the are again advances to a still more illuminated portion of the cathode, the increase in photoelectric cell current is that shown by the distance d betweenthe upper set of horizontal lines. It is seen, therefore, that although the scanning ray has a constant intensity, the increase in photoelectric cell current is dependent upon the illumination of the cathode by the light reflected from the scene. As the ray is projected over the cathode in any desired configuration, it illuminates the light and dark areas with light of constant intensity, but produces a. variation in the photoelectric cell output current, in accordance withthe light and dark portions of the image on the cathode.

The operation of this circuit is identical with that of a direct scanning system in which the increase in photoelectric cell current is determined by the intensity of the reflected light from a directly scanned object, that is, the lightportions of the object will reflect morelight and produce a higher value of photoelectric cell current. The present system, however, instead of projecting the light to great distances with the extreme loss in intensity through distance, has a substantially constant scanning range with nolossoi scanning power due to the varying length of the light projection paths. Inthis way the system is extremely efficient for the scanning of 1 efiiciency is maintained distant scenes, and this in the scanningof near objects. The direct scan ning of an image on an ordinary screen does not produce the photocell current contrast values. as will the present system..

In Fig. 2 of the drawings, a cross section of a television camera is illustrated, with its external regeneration circuits. In this figure, a casing 40 encloses a photoelectric cell 4|, a scanning arc screen 22 with respective lenses .43 and 44a. The photoelectric cell 4| is shown with a curved cathode 46 and anodes 4,4 and 45, arrangedon opposite sides of the cathode, and conforming to the curvature of the cathode. The cathode in this cell is translucent, and will transmit light through it as well as being able to hold the image of a scene projected thereon intercepting sufficient light for this purpose. The advantage. of the curved cathode is that the projection path of the ray of light from the rails 41 of the scanning system 42 is always a constant distance from the cathode and image thereon. In this manner the intensity of the normal light is maintained constant at the image. Thedouble anode arrangement of this system provides a more efficient photocell, since the cathode may be made translucent with active material on both sides thereof. This arrangement is also advantageous from the mechanical point of View. ,An object O is projected on the left hand side of the cathode, while the scanning ray of the screen 41 is projected on the right hand side.

The photoelectric cell is mounted on. an adjustable stand 58, while the scanning apparatus is mounted on a. similar stand 5|, the adjustment of the elements of which is controlled by the thumb locking screws 52 and 53, respectively. The scanning apparatus is shown with a cooling jacket having a fluid passage 54 and heat dissipating material 55 such as steel wool and the like, surrounding the jacket. Heat conveying fluids pass through the fluid channel to maintain the screen electrode rails ll at a constant temperature. A field winding in series with the rail electrodes ll is also cooled by the jacket system.

The external circuits of this system include a photoelectric cell amplifier having either two tubes or a single tube with a double anode, as shown. For instance, the grid fill is connected to the oathode of the photoelectric cell, while the anode M of the cell is connected through a variable inductance 6 to an anode 62 of the vacuum tube. Similarly, the anodel of the cell is connected through a variableinductance 63 to the anode M ofthevacuum tube, A resistance 65 is connected in shunt to these elements for controlling the operation of the cell. A plate potential 66 for the vacuum tube, supplies also the operating voltage for photoelectric cell through the primary of the output transformer 61, chokes 68 and 69, in parallel, variable inductances BI and 63 in parallel, anodes it? and $5 in parallel, cathode Q3, and grid (it. An increase in emission of the cathode to either of the anode-s of the cell will produce an increase in the voltage on the input circuit of the tube, which will be transmitted through the output transformer 61 to an amplifier Hi. The output circuit has a variable inductance ll connected therein which is employed for feed-back purposes. The inductances, therefore, El and 6t and El may be used to produce a certain amount of regeneration. The scanning arc is supplied from a scanning generator 15 through conductorslt. The output of the amplifier is is alsoconnected to conductors 176, for the purpose of increasing the intensity of the arc as the photoelectric, cell currents are increased. This action will produce regeneration by increasing the intensity of the scanning ray in proportion tothe increase in photoelectric'cell current. That is, should the current be considerably increased by the scanning ray falling on a highly illuminated area of the cathode, this increase will be augmented hythesubstantially simultaneous increase in the scanning ray intensity. When the ray falls on a less illuminated area, 'the contrast will be much greater than with the normal scanning ray.

The photoelectric cell currents and the are producing currents are segregated by a filter I? in the output circuit of the system. The output terminals it may be connected to transmission apparatus such as shown at H5 in Fig. l. scanning generator a direct current source, it is unnecessary to employ a filter.

The above described television scanning system is particularly adaptable to the transmission of distant scenes, but it is also particularly efiicient for the transmission of details of near objects. By using a special focus lens, an extremely clear cut image can be formed on the cathode of the celLwhich when scanned by an arc of constant intensity with regeneration, will produce especially fine definition in a reproduced image. In this system there is no distortion in the photocell currents caused by the varying length of projection path for the scanning ray, such as exists when objects are scanned directly.

The non-linear characteristic of a photoelectric cell is adaptable for other uses outside the art of television, and the invention is to be limited only by the scope of the appended claims.

What is claimed is:

1. In a television system, a light sensitive device having a cathode, means for focusing an optical image of a scene to be transmitted on said cathode, and means for scanning with a ray of light said optical image while it is focused on said cathode.

2. In a television system, a photoelectric cell, an electrodynamic arc screen, an amplifier for With the the currents generated by said photoelectric cell, and means for feeding a portion of said photoelectric cell currents to said are screen.

3. In a television system, a camera, a photoelectric cell having its cathode placed at the focus of said camera, means for scanning an object focused on said cathode with a light ray, and means for transmitting the varying electric currents transmitted by said photoelectric cell.

4. In an electrical transmission system, a television camera, said camera including a light sensitive device and a scanning screen, and means for simultaneously focusing the light from said scanning screen, and the reflected light external of said camera on said light sensitive device.

5. In an electrical transmission system, a photoelectric cell, and an electrodynamic arc V scanning screen for scanning said cell, said photoelectric cell having a curved cathode to maintain the projection path of light from said scanning screen constant for every position of said scan ning light.

- 6. In a television transmitting apparatus, a photoelectric cell having a translucent cathode, anodes positioned on opposite sides thereof, means for scanning one side of said cathode with a visible light beam, and means for focusing on the other side of said cathode the scene to be transmitted over said television system.

7. In a television transmission apparatus, a photoelectric cell, scanning means for scanning one side of the cathode of said cell, means for focusing an object to be transmitted on the other side of said cathode, means for amplifying the currents generated by said scanning means, and means for impressing a portion of said generated currents on the light producing means for said scanning system.

8. In an electrical generator system, a photoelectric cell having a cathode and an anode, an amplifier having its input connected to said cathode and anode, means for focusing an image on said cathode to generate a certain value of current in said cell, and means for producing variations in said current value, said means in cluding a constant light ray source.

9. An electrical generator system in accordance with claim 8, in which said last mentioned means comprises an electrodynamic are light scanning system.

10. In a television transmission system, a photoelectric cell, means for focusing the reflected light from a scene to be transmitted on said cell to produce a definiteelectron emission in said cell, and means for Varying said emission in different proportions in accordance with the variations in light intensities on said cell, said means including a luminous electrical discharge source of constant intensity.

11. In combination, an electronic device having an electron emissive surface activated by light, means for projecting on said surface nonuniformly distributed light of a substantially constant value, and means for uniformly increasing the light on consecutive unit areas of said surface.

12. A combination in accordance with claim 11, in which said last mentioned means comprises a television scanning system including an electrodynamic discharge screen.

13. In a television system, a photoelectric cell, a scanning device for generating activating light for said cell, said device comprising an electrodynamic arc in a magnetic field electrodes for determining the path of said arc, and means for cooling said device, said means including channels positioned adjacent said electrodes for housing a circulatory heat conveyor.

14. In a television transmission system, a light sensitive device having a cathode, means for focusing reflected light from a scene to be transmitted on said cathode of said device'to produce a definite electron emission, and means for simultaneously scanning with a ray of light an optical image of said scene formed on said cathode by said focused reflected light while said optical image is on said cathode to cause actuation of said cathode approximately in accordance with the square law.

15. In a television transmission system, a light sensitive device, means for focusing the image of a scene to be transmitted on said device, said image generating electrons at a point on the charactrical currents proportional thereto with a lightsensitive device, comprising combining the currents produced by the light from the entire image projected on said device with the current produced by a unit area scanning beam scanning said image, the total current being determined by the light intensity-output current characteristic of said device.

18. In a television system, a photosensitive device, means for projecting an optical image on said device, means for simultaneously scanning with a ray of light said optical image while it is projected on said device to produce electrical currents proportional to the light densities of the unit areas of said image, the variations in said currents being determined by the characteristic of said device and means for biasing each unit area of said device with light producing said image.

19. A method of television which comprises first energizing the photoelectric surface of a limited area, then sensitizing the said photoelectric "surface with an image of a View, simultaneously traversing said surface with a scanning beam of strong illumination and causing the photoelectric values to be impressed upon a transmitting medium and repeating these steps in the transmission of each individual image.

20. In a television system, a light sensitive device, means for projecting an optical image of an object on said device, and means for scanning with a ray of light said optical image while it is projected on said device.

21. The method of transmissing images of objects electrically with a light sensitive device, comprising projecting an optical image of said object upon said device and scanning with a ray of light said optical image while it is projected on said light sensitive device.

22. The method of transmitting pictures of objects electrically, comprising obtaining an optical image of said object on the cathode of a photoelectric cell, scanning with a ray of light said optical image while it is obtained on said cell, and transmitting the variations in current produced in said cell by said scanning ray.

23. In a television system, a light sensitive device, means for obtaining an optical image of an object on said device, and means for scanning with a ray of light said optical image While it is 5 obtained on said light sensitive device.

24. The method of transmitting images of objects electrically with a light sensitive device, comprising obtaining an optical image of said object upon said device and scanning with a ray of light said optical image While it is obtained on said light sensitive device.

ALEXANDER McLEAN NIC'OLSON. 

